Which of your once deeply-held beliefs have changed as you have gotten older, and why?

I’ll start with my own example: when I was younger, and before I really understood anything about it, I used to think that pro-legal marijuana activism was misguided at best and promoted criminal behavior at worst. My views on the issue have completely turned around. I think that criminalizing marijuana (and indeed, the criminalization of all drug use) has hurt more people than the drug itself. I have also come to think that people should not be denied the personal freedom to use whatever drug they want. And I’m outraged that we have for so long been deprived of the ability to learn about marijuana’s endless medical and therapeutic applications.